[PATCH] net: gue: reject invalid REMCSUM offsets

The REMCSUM option carries an absolute checksum start and checksum field
offset. gue_remcsum() passes them to skb_remcsum_process(), whose
partial path stores offset - start in the u16 skb->csum_offset. If
offset is less than start, this underflows (for example, 1/0 becomes
0xffff).

A forwarded packet can retain CHECKSUM_PARTIAL and reach a
NETIF_F_HW_CSUM driver which trusts the metadata, leading
skb_copy_and_csum_dev() to write two bytes about 64 KiB beyond the
destination buffer.

Reject reversed tuples in both normal and GRO receive paths.

Fixes: fe881ef11cf0 ("gue: Use checksum partial with remote checksum offload")
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <[email protected]>
---
 net/ipv4/fou_core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv4/fou_core.c b/net/ipv4/fou_core.c
index ab09dfcdecbd..c4b8b7293994 100644
--- a/net/ipv4/fou_core.c
+++ b/net/ipv4/fou_core.c
@@ -97,6 +97,9 @@ static struct guehdr *gue_remcsum(struct sk_buff *skb, struct guehdr *guehdr,
 	size_t plen = sizeof(struct udphdr) + hdrlen +
 	    max_t(size_t, offset + sizeof(u16), start);
 
+	if (unlikely(offset < start))
+		return NULL;
+
 	if (skb->remcsum_offload)
 		return guehdr;
 
@@ -308,6 +311,9 @@ static struct guehdr *gue_gro_remcsum(struct sk_buff *skb, unsigned int off,
 	size_t start = ntohs(pd[0]);
 	size_t offset = ntohs(pd[1]);
 
+	if (unlikely(offset < start))
+		return NULL;
+
 	if (skb->remcsum_offload)
 		return guehdr;
 
-- 
2.47.3